It seems everyone was at it. Neanderthals interbred with another species, the Denisovans, as did some of us. Some people from South East Asia have up to 6% Denisovan DNA.
Even Africans whose ancestors never left the continent carry some Neanderthal DNA, because 3000 years ago people from Europe and Asia migrated to Africa. Many modern Africans have inherited some genes, including some Neanderthal ones, from these people.
Now some scientists are going even further. They propose that our entire species is the product of hybridisation between species, and that we owe much of our success to this very fact.
You might not like this idea, in which case it`s worth bearing in mind that hybridisation is common in nature.
Brown bears and polar bears can successfully interbreed when they meet. Most of the Galápagos finches are the result of interbreeding, as are many primate species like baboons and gibbons.
"Seven to 10% of all primate species hybridise, which is common considering a lot don`t ever come into contact with each other," says Rebecca Ackermann of the University of Cape Town in South Africa.
In July 2015 it emerged that a hybrid coral is doing better than either of its parent species. It can survive in a busy shipping channel, which its parents cannot do.
In the past five years, evidence of separate interbreeding events between modern humans (Homo sapiens) and our early ancestors has increased.
In June 2015, researchers announced that a 40,000-year-old skeleton from Romania had the most Neanderthal DNA of any human analysed to date. The "Oase individual" inherited between 6% and 9% of his DNA from the Neanderthals.
Furthermore, the team found that his Neanderthal ancestor lived only 200 years before his death. The genetic evidence confirmed something that anatomists had previously suggested: the Oase individual`s jawbone had some clear Neanderthal traits.
At the time, co-author David Reich of Harvard Medical School in Boston, Massachusetts, US, told BBC News that he was taken aback by the discovery. "In the last few years, we`ve documented interbreeding between Neanderthals and modern humans, but we never thought we`d be so lucky to find someone so close to that event," Reich said.
Intriguingly, the Oase individual did not pass on his Neanderthal genes to modern-day Europeans. Clearly, someone else did, because their DNA is still present.
The Oase individual is just one instance of interbreeding, and more will surely be found as researchers analyse ever more ancient DNA.
Writing in the journal Evolutionary Biology, Ackermann and colleagues argue that hybridisation has worked in tandem with other evolutionary processes "that act to diversify populations".
Hybridisation can also result in new combinations of traits, says Ackermann. "It can be quite a creative evolutionary force, which is not how people thought in the past. Brand-new things could be the product of hybridisation."
Ackermann studies hybrid mice and has found that hybrid lineages often have anomalies in their teeth. There was also a great deal in variation of their size. If these new traits are useful, they can be passed onto future generations.
Of course, beneficial traits can also arise through chance mutations. But if we had to wait for such events we might be waiting for a long time.
Interbreeding can speed up these changes, says evolutionary geneticist Rasmus Nielsen of the University of California, Berkeley in the US. When modern humans left Africa, integrating with other species therefore allowed us to adapt to new environments much more quickly.
For example, the DNA evidence hints that we inherited the ability to fight certain diseases from Neanderthals. When we first arrived in Europe our immune response may have struggled to deal with unfamiliar local diseases, but the offspring of those that interbred with Neanderthals fared better.
The same occurred when Europeans began colonising the Americas, bringing diseases that proved catastrophic to the indigenous population. "The ones that survived were products of mating between Europeans and North Americans," says Nielsen. "Something similar happened, but maybe on a grander scale, between Neanderthal and modern humans."
There is one problem with the idea that hybridisation has been crucial for our evolution. We do not have evidence of any interbreeding between species before modern humans left Africa.
That means some modern humans are known to be hybrids, but not all. Furthermore the known hybridisation events all happened relatively recently, when our species had already evolved to be more-or-less the way it is today.
Of all the many human species that have evolved, only ours has survived to the present day
There is a simple reason why we do not have hard evidence of ancient interbreeding in Africa: we have not yet been able to analyse any DNA that ancient.
However, some researchers believe it did happen. A 2011 study found that some groups of Africans carry genetic material from "an archaic population that split from the ancestors of modern humans". The results are hard to explain without hybridisation, says Nielsen, who was not involved in the study.
Ackermann agrees. There were many species of early human-like creatures, often living alongside each other, one of which eventually gave rise to us. We don`t know who or when, but it seems reasonable to expect that these long-lost species had the occasional romp.
If Nielsen and Ackerman are right, it means we are all hybrids to some extent.
Ackerman goes further, and argues that we owe our cultural success to these ancient meetings.
Of all the many human species that have evolved, only ours has survived to the present day. Our complex culture has clearly been a big part of our ability to outcompete the other species.
Mixing with other species could have helped us develop that culture. When our ancestors met other species, they may have shared knowledge as well as genes.
Learning new habits and tricks from other species could have helped spur on our development. "The things we think of as creativity could be as the result of interaction between different groups," says Ackermann.
We had clearly evolved to be good at copying, learning and innovation, says Simon Underdown of Oxford Brookes University in the UK. These are traits that primed us to become the cultural beings we are today.
Not everyone is convinced that hybridisation has played a key role in this process.
We produced many of our greatest innovations in the past 10,000 years, says Francesco D`Errico of the University of Bordeaux in France. Throughout that period, we were the only human species around. "This implies that innovations emerged in human history for reasons other than from hybridisation."
"We have very little in the way of theory about how human cultural interactions and institutions may have evolved," says Aaron Stutz of Emory University in Atlanta, Georgia, US. So it`s hard to draw any firm conclusions about the role of interbreeding.
One day, possibly within a few years, we might be able to a sequence DNA from much older human specimens. Only then it will become clear just how much hybridisation has shaped who we are.
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